A hydropneumatic suspension system for motor vehicles having large axle load inclinations is described in German Patent Document DE 41 20 758 A1, which is incorporated by reference herein. Double-acting hydraulic cylinders are used in this suspension system, the cylinder chambers of these cylinders on the piston-rod side connected with a first accumulator and the toroid chambers connected to a second accumulator. In this system, a level regulating valve controls the height and a pressure-controlled valve constantly regulates a set pressure ratio between the pressures of the first and the second accumulators as a function of the load on the hydropneumatic actuators. In addition, the pressure control valve is actuated both by the pressure in the pressure line to the cylinder chambers and by the pressure in the pressure line to the toroid chambers.
A more simple embodiment is described in German Patent Document DE 42 42 448 C1, which is incorporated by reference herein, with the advantage that load-sensing pumps can be utilized. Static load changes are also regulated following load changes and subsequent activation and deactivation processes. In this case, a pressure control valve is used to keep the pressure level in the toroid chamber spring circuit constant.
An improvement in convenience is achieved by the apparatus described in German Patent Document DE 197 19 077 A1, which is incorporated by reference herein, and in which a load-matched two-stage pressure control valve is used as the pressure control valve. In this way, it is possible to adjust the suspension system better to the load surges.
In addition, a device is known from German Patent Document DE 101 07 631 A1, which is incorporated by reference herein, and in which a pressure control valve is regulated by a pilot valve actuated by the inlet pressure to the piston chambers and, if the inlet pressure in the inlet line to the piston chambers drops below a specified value, a switch is made to a higher control stage. In this way, the pressure in the toroid chambers of the spring cylinder can be increased in the low load range on the front axle.